1. Field of the Invention
This invention relates to motor operators for electrically positioning the handle of an electrical switch such as a circuit breaker. More particularly, it relates to such a motor operator with a device which prevents repeated unsuccessful attempts to turn on a circuit breaker that has not been reset.
2. Background Information
Electrical switches, such as circuit breakers, typically have a handle by which the contacts of the circuit breaker can be manually opened and closed. They can also be automatically opened, or tripped, in response to currents which exceed defined amplitude/time-characteristics. In many such circuit breakers, the handle must be moved beyond the off position to a reset position following a trip before the handle can be returned to the on position. Often, the handle is spring biased to a position between off and on when the circuit breaker is tripped to provide a visual indication of the tripped condition.
In many applications, a motor operator is provided to position the circuit breaker handle. The motor operator makes it easier to operate large circuit breakers and also provides the capability for remote operation of the circuit breaker. An example of a common type of motor operator is described in U.S. Pat. No. 5,196,658. Such motor operators have a carriage which engages the handle of the circuit breaker. The carriage is reciprocally driven to move the handle to the on and off/reset positions by a threaded shaft which is rotated by an electric motor in response to the operator's activation of a latched relay which energizes the motor. The power circuit for the electric motor includes on and off limit switches which are actuated when the carriage has driven the handle to the on and off/reset positions respectively to terminate energization of the motor. In certain cases, the motor does not have sufficient torque to reset the circuit breaker when starting from the tripped position. In these cases, the handle should be first moved to the on position so that the motor operator generates sufficient inertia to reset the circuit breaker. If the circuit breaker is not reset, the springs biasing the handle to the intermediate tripped position cause the carriage to rebound from the off/reset position toward the trip position by a distance which results in deactuation of the off limit switch. This results in reenergization of the motor with a polarity which again drives the carriage toward the off/reset position. Again, this results in reactuation of the off limit switch to deenergize the motor, but since the motor lacks sufficient torque to reset the circuit breaker, the carriage again rebounds deactuating the off limit switch. Thus, the motor repetitively tries to reset the breaker eventually causing burn-out of the motor. patent application Ser. No. 08/699,305 cited above, addresses this problem and provides an override which maintains the off limit switch in an activated condition if the carriage rebounds from the off/reset position toward the trip position.
A related problem has been identified where a circuit breaker fails to reset and the motor operator is activated to turn on the circuit breaker. If the circuit breaker is not reset, the springs biasing the handle to the intermediate tripped position cause the carriage to rebound from the on position toward the trip position by a distance which results in deactivation of the on limit switch. This results in reenergization of the motor with a polarity which again drives the carriage towards the on position. This can repeatedly occur, and thus can similarly result in burn-out of the motor.
There is a need therefore for an improved motor operator for operating the handle of electrical switches such as circuit breakers.
More particularly, there is a need for such an improved motor operator which does not permit cycling of the motor operator in either direction if the circuit breaker does not reset.